1. Field of the Invention
This invention relates to a pre-drive circuit used in a switching regulator, a DC-DC converter or the like.
2. Description of the Related Art
A pre-drive circuit, one example of which is shown in FIG. 3, is widely known and is adapted to turn the current that flows into the primary side of a pulse transformer on and off, apply the pulses generated on the secondary side of the transformer to a MOS-type field-effect transistor (hereinafter referred to as a "FET") for power, and exercise control to turn a load current on and off by the power FET.
In accordance with the conventional circuit shown in FIG. 3, the secondary side of a pulse transformer T is connected directly to the gate and source of a power FET Q1, and the power FET Q1 is subjected to turn-on/turn-off control by using a drive transistor Q2 connected to the primary side of the pulse transformer Q2. The power FET used in this pre-drive circuit naturally has a large electrostatic capacity (input capacity) as seen from the gate since the current controlled is large.
In FIG. 3, a switching circuit 1 controls a current supplied from a power supply PS to a load L by means of the power FET Q1. This circuit is connected to the pre-drive circuit 2. When the switching transistor Q2 is turned on, the gate of the power FET Q1 is charged to the positive side by the voltage whose polarity is indicated by the solid line, so that the power FET Q1 is turned on. When the switching transistor Q2 is turned off in order to turn off the power FET Q1, the charge stored in the gate of the power FET Q1 becomes a current I.sub.E, which is indicated by the broken line, and is discharged into the secondary coil of the pulse transformer T. With further charging on the negative side, the power FET Q1 is turned off. ZD1 and ZD2 denote protective Zener diodes which limit the voltage applied to the gate of the power FET Q1 so that this voltage will not become excessive.
When the power FET Q1 is turned off after being turned on in the conventional pre-drive circuit of the type described above, it is necessary to preset the magnetic flux of the pulse transformer T to an initial value and charge the gate to a negative voltage required to turn off the power FET Q1. Therefore, the larger the input capacity of the power FET Q1, the shorter the elapsed time from turn-on to turn-off of the power FET Q1 and the greater the change in gate voltage with respect to time, the larger the current I.sub.E must be which flows through the path comprising the source and gate of the power FET Q1. Accordingly, the core of the pulse transformer T is required to be correspondingly large.
In the end, the energy charged in the gate of the power FET Q1 is lost. Consequently, it is meaningless to charge the gate into an unnecessary voltage region beyond the negative voltage that turns off the power FET Q1. Moreover merely increasing the driving power lowers the efficiency of the driving power.
As set forth above, it is necessary that the magnetic flux of the power transformer be rapidly reset to an initial value for the next drive cycle. However, since the gate of the power FET Q1 is charged and connected directly to the secondary side of the pulse transformer T, as mentioned above, time is required for the electric charge resulting from the charging operation to discharge. Hence the flux of the pulse transformer T cannot be rapidly reset to the initial value. Consequently, when high-speed switching is made to take place, there is a time delay for the discharge of the above-mentioned electric charge and, hence, normal operation cannot be achieved.